INFLUENCE OF PROCESS CONDITIONS ON THE DRYING RATE OF CASSAVA CHIPS IN A SOLAR SIMULATOR

ABSTRACT

Experiments were carried out in a solar simulator to study the influence of air temperature (25-40°C), air relative humidity (40-80%), air velocity (0.95-2.2 m/s), radiation intensity (0-916 W/m²), and loading density (10-30 kg/m²) on the drying rate of a bed of cassava chips (2×2×2 cm). Well-known thin-layer drying equations were fitted to the experimental data, and the empirical constants were used in a statistical analysis of the influence of process conditions on the drying rate. The air temperature, air velocity, radiation intensity, and loading density influenced the drying rate significantly (p=0.05). The effects of the air temperature and the radiation intensity were attributed to the temperature-dependent diffusion of moisture within the chips, while the effect of the air velocity was ascribed to the resistance to mass transfer at the air-chip interface. Equations were presented to express the empirical constants as functions of the process variables.     

Influence of the Applied Acoustic Energy on the Drying of Carrots and Lemon Peel

The application of power ultrasound could constitute a way of improving traditional convective drying systems. The different effects produced by the application of power ultrasound may influence the drying rate without provoking any significant increase in product temperature. Due to the fact that the effect of power ultrasound is product dependent, the aim of this work was to address the influence of the applied acoustic energy on the convective drying of carrot and lemon peel.

Convective drying kinetics of carrot cubes (side 8.5 mm) and lemon peel slabs (thickness 7 mm) were carried out at 40°C and 1 m/s by applying different levels of acoustic power density: 0, 4, 8, 12, 16, 21, 25, 29, 33, and 37 (kW/m³). The application of power ultrasound during drying was carried out using an airborne ultrasonic transducer (21.7 kHz). Drying kinetics were described considering a diffusion model.

In both products, the application of power ultrasound improved the effective moisture diffusivity (D_e ). The improvement was linearly proportional to the applied acoustic power density. In the case of lemon peel, the effects of power ultrasound were found over all the range tested (0–37 kW/m³), whereas in the case of carrot, it was necessary to apply an acoustic power density of over 8–12 kW/m³ to be able to observe the influence. The more intense effect of acoustic energy in lemon peel drying may be explained by the fact that lemon peel is a more porous product than carrot.     

Evaluating a Solar Dryer for In-Shell Drying of Split Pistachio Nuts

Abstract

A thin-layer forced air solar dryer was designed to study the feasibility of drying pistachio nuts. The dryer was tested during the 2001 and 2002 drying seasons. The maximum temperature in the solar collector reached 56°C, which was 20°C above the ambient temperature. The required drying time was 36 h. During the first day of drying (0800 to 1700 h) the moisture content dropped to about 21% (wb). The final moisture content of the dried nuts was 6% wb, which was 1% below the recommended storage moisture. The drying constant of the pistachio nuts during solar drying was determined using two mathematical models, a one-term series solution of Fick's diffusion equation and an exponential decaying model. There was no significant difference between the two models (α = 0.05). In general, the quality of solar dried nuts was better than the conventional heated air due to slower drying rates.     

Modeling of the Seedless Grape Drying Process using the Generalized Differential Quadrature Method

Mathematical modeling of the grape drying process is important in understanding the transport phenomena involved in the production and processing of dried grapes. Drying models proposed in the literature have simplifying assumptions, and thus ignore important phenomena such as shrinkage and changes in transport properties which occur during the drying process. Consequently, a mathematical model is developed for the seedless grape drying process, which considers the effects neglected in previous models. Since an analytic solution to this nonlinear model is impossible, the generalized differential quadrature method is used to solve the models' equations. The model is validated with experimental data obtained from a laboratory scale convective tray dryer operating at 50–70 °C and an air velocity of 1.5 m/s. Model predictions are in close agreement with experimental data due to the inclusion in the model of shrinkage and variation in moisture diffusivity. Model results can serve as a framework to improve the performance of existing and novel dryers, and also in the design of process simulators for dryers.     

Drying of Potato Slices in a Pulsed Fluidized Bed

This article presents experimental and modeled drying kinetics of potato slices of the Desiree variety (9 × 9 × 3 mm³) in a pulsed fluid bed as a function of the air velocity, air temperature, and rotating disk velocity of the pulse generator. A statistical multifactor experimental design (2³) was applied to analyze the drying process with two levels each of drying temperature, air velocity, and rotating disk velocity. The results showed that the significant factors were air temperature, air velocity, rotating disk velocity, and the binary interactions of air velocity with both the temperature and the rotating disk velocity. The simplified variable diffusivity model (SVDM) gave the least deviation for the experimental data. The effective diffusivity values determined in this work are similar to those reported in the literature.     

Drying of Potato Slices in a Pulsed Fluidized Bed

This article presents experimental and modeled drying kinetics of potato slices of the Desiree variety (9 × 9 × 3 mm³) in a pulsed fluid bed as a function of the air velocity, air temperature, and rotating disk velocity of the pulse generator. A statistical multifactor experimental design (2³) was applied to analyze the drying process with two levels each of drying temperature, air velocity, and rotating disk velocity. The results showed that the significant factors were air temperature, air velocity, rotating disk velocity, and the binary interactions of air velocity with both the temperature and the rotating disk velocity. The simplified variable diffusivity model (SVDM) gave the least deviation for the experimental data. The effective diffusivity values determined in this work are similar to those reported in the literature.     

Effects of Drying Parameters on Heat Transfer during Drying of Fermented Ground Cassava in a Rotary Dryer

The effects of drying parameters on heat transfer during drying of fermented ground cassava in a rotary dryer were studied. The fermented ground cassava was dried in a bench-scale rotary dryer at different inlet air temperatures, inlet air velocities, relative humidities, feed rates, drum drive speeds, and feed drive speeds. It is shown that inlet air temperature, inlet air velocity, and feed rate have significant effects on the specific heat transfer coefficient and heat load in the material. Models that predict the specific heat transfer coefficient as a function of inlet air temperature and inlet air velocity and the heat load as a function of inlet air temperature, inlet air velocity, and feed rate are also presented. Predictions of the models are compared with experimental data and good agreement is obtained.     

Drying Kinetics in a Vertical Gas‐Solid System

A one‐dimensional steady‐state two‐fluid model has been developed to demonstrate the drying kinetics in the vertical up‐flow gas‐solid system. The model takes into account mass, momentum, and heat transfer between the continuous and dispersed phases. A set of non‐linear differential equations have been solved numerically for the velocity, moisture content, and temperature of both the continuous and dispersed phases along the dryer length. The effect of operating parameters on drying kinetics has been critically examined and the model simulations are compared with the data reported in the literature.     

Modeling of Thin-Layer Drying on an Inert Sphere

A semi-empirical model was developed for the drying of press cake on an inert sphere in the spout region of a dryer. The coefficients for the Lewis and Page (for the prediction of moisture ratio) and the Chung-Pfost (for the prediction of equilibrium moisture content) models were determined experimentally. The predicted temperature of the press cake was validated using three trials conducted at drying conditions (temperature, relative humidity): 55°C, 55%; 65°C, 45%; and 75°C, 43%. Predicted temperatures were within ～10% of the experimental temperatures. Improved prediction accuracy was achieved as press cake temperature approached air temperature.     

Mathematical Modeling of the Fluidized Bed Drying of Cubic Materials

The unsteady‐state simultaneous heat and mass transfer between gas and potato cubes during the drying process in a batch fluidized bed was described by a mathematical model. Mass transfer was considered to occur in three dimensions whereas heat transfer between the gas and dried material was assumed to be lumped. It was found that the model could describe the drying process with acceptable accuracy. The moisture profile inside the material at any cross‐section and at any time can be predicted by the model.     

Transfer Phenomena During the Drying of a Shrinkable Product: Modeling and Simulation

Abstract

Drying is an essential step in many production and treatment processes. The control of dry product quality is more and more required in order to insure a good using or handling of the material. Then new mathematical models should integer these requirements and consider the phenomena of shrinkage. This work gives a mathematical formulation of the different transfer phenomena (heat, mass, and momentum) which describe the process. The model tries to be relatively simple but sufficiently complete in order to predict and analyze the distribution of the temperature, the moisture, the strain, and the stress during the process. The product is considered as a two-phase, homogeneous, hygroscopic, isotropic, and highly shrinkable medium. The thickness of the sample is sufficiently small in order to consider the fluxes of heat and mass unidirectional. The principal equations of the model are written in a Lagrangian formulation because of the shrinkage behavior. The model is solved numerically by a finite difference method. A validation of the results is achieved by the comparison of the numerical and experimental data. The simulation allows the derivation of the time and space evolution of several parameters: temperature, moisture, strain, and stress.     

Transfer Phenomena During the Drying of a Shrinkable Product: Modeling and Simulation

Drying is an essential step in many production and treatment processes. The control of dry product quality is more and more required in order to insure a good using or handling of the material. Then new mathematical models should integer these requirements and consider the phenomena of shrinkage. This work gives a mathematical formulation of the different transfer phenomena (heat, mass, and momentum) which describe the process. The model tries to be relatively simple but sufficiently complete in order to predict and analyze the distribution of the temperature, the moisture, the strain, and the stress during the process. The product is considered as a two-phase, homogeneous, hygroscopic, isotropic, and highly shrinkable medium. The thickness of the sample is sufficiently small in order to consider the fluxes of heat and mass unidirectional. The principal equations of the model are written in a Lagrangian formulation because of the shrinkage behavior. The model is solved numerically by a finite difference method. A validation of the results is achieved by the comparison of the numerical and experimental data. The simulation allows the derivation of the time and space evolution of several parameters: temperature, moisture, strain, and stress.     

Drying of Turnip Seeds with Microwaves in Fixed and Pulsed Fluidized Beds

This article presents experimental and modeled drying kinetics of turnip seeds as a function of the type of air-particle contact (fixed bed, pulsed fluidized bed), humidity of the air, and three levels of microwave irradiation. The effects of those factors on the drying time required to reach a moisture content of 0.1 kg/kg d.b. were determined with an experimental design of 12 experiments by using the software Statgraphics. It was found that in drying, a significant factor was the application of microwaves to a pulsed fluidized bed. The effect of the humidity of the air only became noticed when the moisture contents of the seeds was below 0.2 kg/kg d.b. The simplified variable diffusivity model (SVDM) gave the least deviations for the experimental data. The effective diffusivity values determined in this work are similar to those informed in the literature for experiments without microwave application. The application of microwaves in combination with pulsed fluidization of the turnip seeds resulted in a fourfold increase of the effective diffusivity.     

Thin-Layer Drying of Mate Leaves (Ilex paraguariensis) in a Conveyor-Belt Dryer: A Semi-Automatic Control Strategy Based on a Dynamic Model

A feedback strategy of drying control of mate leaves in a thin-layer conveyor-belt dryer was experimentally evaluated. Moisture content in the discharge of the continuous dryer was controlled by manually adjusting the speed of the moving belt between 3.7 × 10^?4 and 15.2 × 10^?4 m s^?1 for approximately 7200 s in 120 s time steps. The sets of PID controller parameters and manipulated conveyor velocities were computed with a dynamic drying model at conditions identical to those found in the closed-loop experiments. The model is represented by a system of two partial differential equations built by energy and solute mass balances in the solid phase of the dryer. A large set of experimental drying curves and temperature of mate leaves as a function of drying time, in the temperature range from 55 to 130°C, confirmed the reliability of the considered model. Experimental closed-loop responses of discharge moisture content in the presence of disturbances in the feed moisture content (≈ 0.5 ? 1.7 dry basis) and variations in set-point (≈ 0.1 ? 1.0 dry basis) validated the suggested control scheme.     

Drying of Turnip Seeds with Microwaves in Fixed and Pulsed Fluidized Beds

This article presents experimental and modeled drying kinetics of turnip seeds as a function of the type of air-particle contact (fixed bed, pulsed fluidized bed), humidity of the air, and three levels of microwave irradiation. The effects of those factors on the drying time required to reach a moisture content of 0.1 kg/kg d.b. were determined with an experimental design of 12 experiments by using the software Statgraphics. It was found that in drying, a significant factor was the application of microwaves to a pulsed fluidized bed. The effect of the humidity of the air only became noticed when the moisture contents of the seeds was below 0.2 kg/kg d.b. The simplified variable diffusivity model (SVDM) gave the least deviations for the experimental data. The effective diffusivity values determined in this work are similar to those informed in the literature for experiments without microwave application. The application of microwaves in combination with pulsed fluidization of the turnip seeds resulted in a fourfold increase of the effective diffusivity.     

太阳能相变储热在木材干燥中应用的初步研究

针对木材太阳能干燥间歇性的不足,在原有太阳能干燥装置的基础上,增设了多管道叉排石蜡相变储热系统。研究结果表明,随着储热系统管排数的增加（5、7、9、11排）,其放热时间逐渐缩短,换热系数呈现增加趋势,平均值分别为17．8、22．1、24．7、26．4W／（m^2·℃）;在其它参数相同的条件下,当管排数不大于5时,储热温度对换热系数的影响不大,但当管排数大于11,储热系统显热储存的热量就不能够忽略;在其它参数相同的条件下,随着风速的增加,换热系数呈现增长趋势,风速为1．0、1．5和2．0m／s时,放热速率的比值为0．89：0．94：1。     

Trends in Microwave-Assisted Freeze Drying of Foods

Microwave-assisted freeze drying (MFD) can be accomplished in two distinct ways: freeze drying assisted concurrently with microwave application (MFD-1) and freeze drying and assisted microwave/vacuum microwave drying in two consecutive separate drying stages (MFD-2). MFD is a rapid dehydration technique that can be applied to certain foods, particularly to seafoods, solid soup, and fruits and vegetables. MFD involves much less drying time and energy consumption than conventional freeze-drying methods. Currently, this technology has been successfully used to dry many food materials and has potential in the food industry. Increasing concerns over product quality, energy savings, and production costs have motivated researchers and the industry to adopt MFD technologies. The advantages of MFD include shorter drying time, energy savings, improved product quality, and flexibility in producing a wide variety of dried products. However, current applications are limited to small categories of foods due to high startup costs and relatively complex technology compared to conventional freeze drying. This article presents a concise review of recent progress in MFD R&D and makes recommendations for future research to bridge the gap between laboratory research and industrial applications.     

压降变化对干燥系统影响的研究

影响干燥系统正常工作的因素很多,本文针对几种对流干燥装置压降变化对干燥系统的影响进行了测试由测试,结果分析可看出,系统某部分压降变化对干燥系统的影响极大,并得出干燥系统在较好状态下的静压零点的位置对于干燥系统的设计及生产调试具有指导意义。     

Assessing the Influence of the Drying Schedule on Moisture Variability in the Batch Drying of Timber

Abstract

Timber drying schedules influence the dispersion in moisture content between boards at the end of drying. Both a single set-point schedule and a double set-point schedule are selected to illustrate the concept. A simple deterministic drying model that can predict moisture content as a function of time for a single and double set-point schedule is presented. Given random variability in both the initial moisture content and drying rate of the product, theoretical expressions for the range in moisture content as a function of time are developed. The predictions of these equations are compared with experimental measurements from a laboratory kiln. A general methodology whereby the basis of commercial schedules can be analyzed as to their performance with respect to minimizing product moisture dispersion is constructed. The approach should prove useful in assisting drying process designers, concerned about moisture variability, in distinguishing between otherwise equal schedules.     

Estimation of the Effect of Shape and Temperature on Drying Kinetics Using MLP

The kinetics of drying is described in the article using artificial neural networks (multilayer perceptron MLP). Drying curves for vegetables are possible to obtain theoretically on the basis of the equations of mass transfer in a porous material. A key role in these equations is played by the effective coefficient of water diffusion in the form of liquid, vapor or jointly as liquid and vapor. The diffusion coefficient which depends both on moisture content in the material and temperature should be determined experimentally. The drying kinetic curve in this article is treated as a time series dependent on the state of material prior to drying and on the constant K characterizing process parameters such as drying temperature and describing the material, e.g., its shape and moisture content. Constant K characterizes the analyzed material from the drying point of view because it contains a diffusion coefficient and depends on the shape of material. The following materials were analyzed: beetroot and potato dried in the form of cubes, slices, chips, and strips. Experiments were carried out in a laboratory oven dryer at process temperature 50, 60, 70, 90, and 106°C.